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Heavy Machinery loads on Slab on Grade design 6

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jbuening

Structural
Feb 15, 2010
44
Does anyone have any insight or tips on a slab on grade design for heavy mining trucks? For example, a mining truck manufacturer needs a parking lot designed for their finished heavy machinery and one of their trucks has a single wheel load of 62k but is spread in 13.2 ft^2 contact patch (large tires). That results in about 4.7 ksf pressure. I have other ones as well as a tandem trailer with 150k load to analyze. That company typically uses 12" thick slab with #5 bars in each direction and on 6" of agg subbase, but I'd like to determine if this section will work.

I have the ACI Slabs on Grade manual but it doesn't address that kind of contact area size. Those are designed for forklift or standard truck traffic it seems. I also have the Army Manual TM 5-809-12 and it does a little better, but they base the loadings on Design Index, with 10 being the highest (120k track laying vehicles). Nothing really seems to fit the bill with the Army Manual, but a DI of 10, Subgrade modulus of 150psi, flexural strength of 530psi results in a slab thickness of about 8.5". I don't exactly have a track vehicle though.

Slabs on Grade have always seemed like black magic to me, as there are tons of charts in the ACI/Army publications but no real equations to show how those numbers were determined. When you are outside the chart limits, no direction is provided. Thoughts?
 
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Could you check with your local DOT pavement engineers?

 
"Slabs on Grade have always seemed like black magic to me" - I am in the same boat.
What I found helpful though is making a quick FE model of SOG. Just don't forget to apply loads at the corners and edges of the slab. As expected, results would be very sensitive with respect to the soil modulus K. If you dont have K value you can find estimated ranges for various soil types in the literature (like Bowles, etc).

For certain types of soil you may need to increase the subbase and/or slab thickness, or to provide top and bottom rebar. Cost analysis will tell what is the most feasible option.
 
Thanks. Local DOT won't touch it since its private work unfortunately. They don't deal with this kind of loads either. I finally found at the end of the ACI book that "unique situations outside the charts of this book will require a FEA to design the slab". Exactly as sasa2k said. I also found within that book that there are no equations that the charts were derived from, but rather a FEM slab program was used to develop the charts.

 
This slab is huge (area-wise) and will have joints at 15' spacings but there will be dowels at the joints and the slab will have temp/shrinkage bars. How does everyone determine the extents of the FEA model for the slab size? Should I model it as a 15' square and apply loads at the edges and at the center, or should I consider a larger area since there are load transfer devices at the joints? No expansion joints will be used except around the perimeter of the slab, where it meets with existing.
 
It is my understanding that RISAFoundation can handle patch loads on SOG's. My next thought is runway design. There must be a procedure used by the aircraft industry.
 
I only have StaadPro, if that makes a difference :(
 
jbuening

FEA can be done in STAAD you just have to read the manual and..... you may be out of your element so you may need to seek guidance with someone who is at least familiar with designing systems using FEA.

I have messed around with FEA in STAAD before but have never used the results because of the item discussed above.

Splitrings

If I remember correctly from my studies (a long time ago), runway design was more about the sub-base than it was about the topping. I seem to remember them being ridiculously thick (including the sub base).

 
Regarding slab design of heavy loads, I suggest you either follow the limiting stress approach (typical of ACI 360, chapter 7) and ignore reinforcing (reinforcing is for crack control only, not for strength),
OR
you design using active reinforcing - essentially a conventional reinforced concrete design using ACI 318 considering factored shears and moments, almost like an elevated slab, except with soil springs (where you take the upper and lower bound to capture expected behavior).
I always prefer the latter, but I think the slab gurus generally prefer the former (they argue more akin to pavement design, but really more like voodoo).
 
I've got another document titled "Rigid Pavements for Air Fields" but I'm having trouble uploading it. It's Army Technical Manual TM 5-825-3. I'll keep trying to upload the file.
 
Thanks everyone for the tips thus far. I actually don't have the ACI 360, apparently the one I have is PCA - Concrete Floors on Ground. Guess I'll invest in the ACI version!

CTW, I already have that one, which is the Army/Air Force manual. Unless you know more than I about the manual, it divides the loading into Design Indexes 1-10. DI 10 is essentially a 120k track-type vehicle, but mentions nothing about track size. I've seen track-type equipment with 15' long and 2' wide tracks, so that 120k is distributed into a larger area. I guess the part I'm struggling with is relating all these charts to the type of vehicles I'm working with. One vehicle has a wheel contact patch of 43.6"x43.6" and load of 70k, which is at the high end of area contact patch. Another is a truck tandem axle trailer with wheel load of 27.5k but a standard 20"x20" contact patch. The Army manual divides it up into either forklift or track-laying vehicles. I'll look into the other one for Air Fields to see what it says.

My gut feel says the 12" slab on 6" agg subbase is sufficient, but need to back that feeling up with calcs. I will get the ACI 360 and go with the limiting stress approach.
 
Sorry, I didn't mean to upload TM 5-809-12. I must have accidentally grabbed that one. The other document that is linked in my first post has some good information. Check out Rigid Pavements for Air Fields before you purchase ACI 360.
 
I have always taken a different approach to this type of design. For a slab like you are looking at I would design it using Roark's Formulas using Table 26 Case 13. This is a continuous plate supported continuously on an elastic foundation of soil modulus "k". This is essentially what all of the mentioned documents are doing in one method or another. I look at my tire loads and patch areas then convert that to an equivalent circle for the circular patch loading. You can use the ACI document to get your allowable concrete stresses but you are more or less limiting yourself to an infinite fatigue life for the bending stress. I have designed many slabs for your type of application using this method and they have all been successful. I also use the reinforced method for the joint spacing so that the joints can be spaced out more. The customer's spec on the slab and rebar would be on par with what I would typically have with any type of subgrade strength at all.
 
Hmmm, thats a book that I don't have (Roark's Formulas). Aggman, is that something that can be regurgitated here or would I have to buy the book?
 
You need to buy it in my mind. It's a vital piece in any good structural engineers library. I use mine weekly. It's essentially the winkler foundation theory though.
 
There is a new edition of Roark's coming out in September. Chk here:


Combination of Army Technical Manual for preliminary sizing and Roark's Formulas to verify FE results should be sufficient.

About FE - you can use STAAD, however I found it very cumbersome. You can model only a typical area including joints and apply the appropriate boundary conditions to simulate the rest of the structure (or just model a complete SOG - computers are fast enough noways for this type of analysis). At joint location you need to release the moments. Support all the nodes on springs (constant equal to k x tributary area). Most of the modern FE software will do this automatically. Apply pressure loads (area equals to contact patch area) at various locations including center, edges, and corners.
You may find that when the load is at the corner or edge of the slab top rebar is required.

BTW - I remember there was an example in the STAAD manual that describes modeling SOG.
Cheers
 
Wow, good stuff everyone! I think I got it down to a reasonable analysis, but keep the information coming if you have it...as it will benefit me in the future as well as many others.
 
Bringing this one back up. The question has came up about expansion joints. Keep in mind this is a very large (600'x400' or so) outdoor parking lot for large mining trucks, subjected to midwest temperature swings. The slab is 12" thick with temp/shrinkage and contraction joints at 15' centers each direction. This parking lot will butt up against an existing parking lot on all four sides. There is absolutely nil that I can find in any ACI/PCI/Army manual about expansion joints, their needs, and what sizes are needed. They each have a short paragraph on them, stating they aren't typically needed for indoor slabs.

Should I provide an expansion material like PJF between the existing and proposed slab? If so, I should also use load transfer dowels with the end into the existing debonded correct? What formula do you typically use to determine the thickness of the PJF, if provided? Bridges, which I'm more accustomed to, have a definite expansion and we typically use 0.0000065/°F with 80° swings. Using the 600' length I'd have 3.75" total movement, meaning 1.875" on each end. This doesn't account for expansion of the adjacent slab though.

Any guidance you can provide would be appreciated!
 
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